1. Field of the Invention
The present invention relates to multi-line wireless telephone systems and, in particular, to receiver subsystems in such systems designed to synchronize when communicating between transceivers of the system.
2. Description of the Related Art
The use of telephones and telephone systems, including wireless telephone systems, is widespread. In wireless telephone systems, such as digital 900 MHz systems, a cordless or wireless telephone handset unit communicates via either analog or digital radio signals with a base unit, which is typically connected via a standard telephone line to an external telephone network. In this manner, a user may employ the wireless handset to engage in a telephone call with another user through the base unit and the telephone network.
Multi-line wireless telephone systems are also in use in various situations, such as businesses with many telephone users. Such systems employ a handset that communicates with up to N handsets simultaneously, typically with digital communications schemes, such as time division multiple access (TDMA). It is desirable to implement the features of current private branch exchange (PBX) systems in a multi-line wireless telephone system.
Wireless telephone systems, such as digital 900 MHz systems, are typically used to establish a wireless network between the base unit and several handsets. The base station transceiver must synchronize with each active handset. This synchronization can require different demodulation and related parameters for each handset, since some handsets may be closer to the base station than others, giving rise to a large dynamic range difference between each handset-base station pair. For example, since the handsets and base station have independent local oscillators for frequency references, and signal strengths, there will be carrier offsets and other demodulation parameters unique to each handset to station link. In addition, the system may be located in an indoor office environment, giving rise to unique multipath problems for each handset-base station link which need to be addressed differently for each handset-base unit pair.
One way to address this problem is to use an independent link for each handset. However, this approach is not without its own problems. For example, with over 100 dB differences between close and far signals, it is difficult to use FDMA (frequency division multiple access) techniques to establish the independent links. Similarly, CDMA (code division multiple access) methods require extremely tight power controls and/or many chips per symbol.
A time-division system like TDMA is often used in multi-line wireless phone systems. In such a system, a single RF channel is used, and each handset transmits and receives data during a dedicated time slice within an overall cycle or epoch. In such a system, however, it can be difficult for the base unit to adequately synchronize with each handset at the beginning of its respective time slice, since each operates on a link having potentially different characteristics than those of the other handsets. One possible solution is to use an extremely fast receiver that is able to re-synchronize at the beginning of each time slice, even if each time slice corresponds to a handset having a link with different characteristics than the last. This solution, however, requires a very fast receiver, with its attendant cost and complexity, and is not always the most efficient way to synchronize links.
Previous TDMA communications systems are described in PCT Application No. WO 96/34481 (Ionica International Limited), published Oct. 31, 1996, and PCT Application No. WO 97/13388 (Telefonaktiebolaget), published Apr. 10, 1997. The Ionica reference describes a telephone system which synchronizes with a unit in accordance with various demodulation parameters, and the Telefonaktiebolaget reference describes various aspects of modulation and time slot allocation in a TDMA type cellular telecommunication system.
A wireless telephone system comprises a base transceiver having a base receiver and a plurality of wireless handsets. Each handset comprises a handset transceiver for establishing a time-division multiple access (TDMA) link over a shared channel with the base unit via the base transceiver, in which each handset communicates during an exclusive time slice of a TDMA scheme that allocates time slices to handsets. The base receiver is characterized by a plurality of demodulation parameters and is for synchronizing with each handsets. The base receiver stores a set of demodulation parameters for each handset and switches to the set of demodulation parameters for a particular handset when the base unit is to synchronize with the particular handset.